This invention relates to a picture signal processing apparatus for level compressing high-luminance portions of picture signals, a color video camera carrying the apparatus and a picture signal processing method.
In picture signal standards of standard television systems, such as National Television System Committee (NTSC) or Phase Alternation by Line (PAL) systems, the black level and the maximum white level of luminance signals are fixed, such that a portion brighter than the maximum white level cannot be displayed. Thus, in video cameras, luminance compression is used in order for the high luminance portion to be accommodated in picture signals. That is, with the white level in the picture signal standard of 100%, the maximum white level is 105 to 110%, so that image pickup signals of the luminance signal level of 95 to 200% obtained from an imager are compressed in level so that the imaged signals with the luminance signal level of 90 to 200% obtained from the imager will be accommodated in the luminance signal level of from 95 to 110%.
An illustrative inner structure of a video camera 9 handling monochomatic picture signals is shown in FIG. 1.
The video camera 9 shown in FIG. 1 includes a CCD image sensor 2 on which falls an imaging light beam from an object via an imaging lens 1. The CCD image sensor 2 images an object image formed by the imaging light for conversion to electrical signals. The electrical signals obtained by the CCD 2, that is the imaging signals, are processed by a contour enhancement circuit 3 with contour enhancement in both the horizontal and vertical directions. The electrical signals are also processed by a luminance compression circuit 4 with luminance compression.
An output signal of the luminance compression circuit 4 is processed by a gamma correction circuit 5 with non-linear correction known as gamma correction and is further processed by a white clipping circuit 6 with white clipping for prohibiting outputting of a signal higher in luminance signal level than the video signal standard. The resulting signal then is amplified by an amplifier 7 so as to be used as an output video signal of the video camera 9.
If the signal level of a luminance signal Y prior compression by the luminance compression circuit 4 is y, the signal level of the luminance signal after compression is yxe2x80x2 and a signal Y for which the signal level y exceeds the signal level Kp is compressed with a compression ratio Ks, the luminance compression by the luminance compression circuit 4 is given by the following equations:
yxe2x80x2=y, if yxe2x89xa6Kp
yxe2x80x2=Ks(yxe2x88x92Kp)+Kp, if y greater than Kp.
For example, if the 100% white level in the above image signal standard is 1, the luminance signal Y prior to compression, having the luminance signal level up to 200% (=2) is suppressed to the luminance signal level of 110% (=1.1), and the signal of Kp is 95% (=0.95),
Ks=(1.1xe2x88x920.95)/(2xe2x88x920.95)
such that
yxe2x80x2=y, if yxe2x89xa60.95
and
yxe2x80x2={(1.1xe2x88x920.95)/(2xe2x88x920.95)}xc3x97(yxe2x88x920.95)+0.95, if y greater than 0.95.
This luminance compression can be represented by a graph of FIG. 2. If the white level in the image signal standard is 100%, the abscissa and the ordinate in FIG. 2 represent the signal level of the luminance signal Y prior to compression, that is the input signal, in percentage (%), while the ordinate in FIG. 2 represents the signal level of the luminance signal Y after compression, that is the output signal, similarly in percentage
The above-described luminance compression system is herein termed the first system.
Although FIG. 1 shows the structure of the video camera 9 handling monochromatic signals, as an example, the inner structure of the color video camera handling color image signals, shown for example, in FIG. 3, is known.
As differences between the structure of a color video camera 10 shown in FIG. 3 and that of the video camera 9 shown in FIG. 1, a color separation prism 12 for separating the imaging light for forming an image of an object into color components of three prime colors of red, green and blue is arranged at back of an imaging lens 11, while there are provided CCD image sensors 13R, 13G and 13B for imaging each object image separated into respective color components, amplifiers 14G, 14R and 14B for taking white balance of red, green and blue signals R, G and B obtained as imaging signals of respective colors by the CCD image sensors 13R, 13G and 13B, contour enhancement circuits 15R, 15G and 15B for executing contour enhancement or luminance compression for three channels, luminance compression circuits 16R, 16G and 16B, gamma correction circuits 17R, 17G and 17B and while clip circuits 18R, 18G and 18B. In addition, there is added an encoder 19 for converting three prime colors R, G and B to color image signals of, for example, NTSC or PAL.
That is, in the color video camera 10 shown in FIG. 3, the imaging light from an object, incident via an objective lens 11, is separated by a color separation prism 12 into three prime colors of red, green and blue. The light of the respective color components falls on the associated CCD image sensors 13R, 13G and 13B so as to be thereby converted into imaging signals associated with the light of the respective color components, that is into three prime color signals R, G and B.
These three prime color signals R, G and B from the CCD image sensors 13R, 13G and 13B are adjusted in signal levels by associated amplifiers 14G, 14R and 14B in order to achieve the white balance. The resulting signals are sent to associated respective contour enhancement circuits 15R, 15G and 15B for contour enhancement. Output signals of the luminance compression circuits 16R, 16G and 16B are gamma-corrected by the respective gamma correction circuits 17R, 17G and 17B and white-clipped by the associated while clip circuits 18R, 18G and 18B so as to be thence supplied to an encoder 19. The encoder 19 generates color video signals pursuant to the standard television system, such as NTSC or PAL. These color video signals are sent to an amplifier 20. The color video signals, amplified by the amplifier, serve as output video signals of the color video camera 10.
It is noted that luminance compression by the color video camera 10 shown in FIG. 3 is performed by the above-mentioned first system independently for each of the three prime color signals R, G and B. The system for independently luminance-compressing the three prime color signals R, G and B by the above-mentioned first system is herein termed a second system.
However, with this second system, there is raised a problem that the color hue or saturation is changed in high luminance portions obtained on imaging a colored object.
Thus, the present Assignee proposed in Japanese patent application no. H-8-91575, which corresponds to WO98/51089 and U.S. Pat. No. 6,111,607, a level compressing method for color video signals in which luminance signals Y are generated from the three prime color signals R, G and B to perform luminance compression on the basis of the luminance signals Y instead of compressing the three prime color signals R, G and B by independent luminance compressing circuit.
In this level compressing method for color video signals, disclosed in Japanese Patent Application No. H-8-91575, the three prime color signals R, G and B are compressed in luminance using a luminance compressing circuit 150 shown for example in FIG. 4.
In the luminance compressing circuit 50, the three prime color signals R, G and B entering input terminals 51R, 51G and 51B are sent to associated multipliers 56R, 56G and 56B, respectively, while being also sent to a Y-matrix circuit 52.
The Y matrix circuit 52 generates luminance signals Y from the supplied three prime color signals R, G and B. The luminance signals Y generated by the Y-matrix circuit 52 are sent to a high luminance compression circuit 54.
The high luminance compression circuit 54 compresses high luminance portions of the luminance signals Y by the above-described first system in accordance with the above-mentioned first system, using coefficients for Kp and Ks, sent from a terminal 53.
The luminance signals Yxe2x80x2, outputted by the high luminance compression circuit 54, are sent to a divider 55 which then divides the luminance signals Yxe2x80x2 from the high luminance compression circuit 54 with the luminance signals Y from the Y-matrix circuit 52 to find a coefficient k.
The coefficient k found by the divider 55 is sent to the multipliers 56R, 56G and 56B fed with the above-mentioned three prime color signals R, G and B. These multipliers 56R, 56G and 56B multiply the three prime color signals R, G and B with the coefficient k in order to perform luminance compression for maintaining saturation and color hue.
Three prime color signals Rxe2x80x2, Gxe2x80x2 and Bxe2x80x2, obtained as output signals of the multipliers 56R, 56G and 56B, are sent to associated saturation compression circuits 60R, 60G and 60B, respectively, while also being sent to a maximum value detection circuit 57.
The maximum value detection circuit 57 detects the maximum one of the three prime color signals Rxe2x80x2, Gxe2x80x2 and Bxe2x80x2, obtained as output signals of the multipliers 56R, 56G and 56B, as namYxe2x80x2.
This maximum value namYxe2x80x2 is sent to a coefficient processor 59 fed with the luminance signals Yxe2x80x2 from the high luminance compression circuit 54 and with a setting coefficient Wc31 xcex3 specifying the white clip level from a terminal 81. Using these signals, the coefficient processor 59 finds a coefficient kxe2x80x2 by processing:
kxe2x80x2=(Wcxe2x88x92xcex3xe2x88x92Yxe2x80x2)/(namYxe2x80x2xe2x88x92Yxe2x80x2)xe2x80x2.
The coefficient kxe2x80x2, found by the coefficient processor 59, is sent to the above-mentioned saturation compression circuits 60R, 60G and 60B which is fed not only with the three prime color signals Rxe2x80x2, Gxe2x80x2 and Bxe2x80x2, as output signals of the multipliers 56R, 56G and 56B, but also with the luminance signals Yxe2x80x2 from the high luminance compression circuit 54. Using these signals, the saturation compression circuits 60R, 60G and 60B effectuate saturation compression under a constant state of luminance and color hue.
The three prime color signals Rxe2x80x3, Gxe2x80x3 and Bxe2x80x3, obtained as output signals of the saturation compression circuits 60R, 60G and 60B, are outputted at associated terminals 61R, 61G and 61b as output signals of the luminance compression circuit.
If, in the color video camera, having the luminance compression circuit 50 for luminance compression as described above, the lens stop is opened as an object of a given color is imaged, there is no change in the three prime color signals Rxe2x80x2, Gxe2x80x2 and Bxe2x80x2 obtained as output signals of the multipliers 56R, 56G and 56B, until the maximum value namYxe2x80x2, detected by the maximum vale detection circuit 57, exceeds the white clip level as indicated by setting coefficient Wcxe2x88x92xcex3 as set by the coefficient processor 59. If the maximum value namYxe2x80x2 exceeds the white lip level, chromaticity is lowered as the luminance and the color hue are maintained, with the saturation becoming equal to zero when the luminance signals Yxe2x80x2 reaches the white clip level. FIG. 5 shows an example of a signal entering the luminance compressing circuit 50. The maximum value namYxe2x80x2 in this case is equal to the green color signals. In FIG. 5, the abscissa and the ordinate dente the amount of opening of the lens iris, usually provided in an imaging lens of the video camera, and an output signal level, respectively.
The above-described system of effectuating luminance compression based on the luminance signals Y generated from the three prime color signals R, G and B is herein termed the third system.
In the above-described third system, the saturation is not compressed whatsoever until the maximum value namYxe2x80x2 reaches the white clip level, so that significant saturation of the prime color is directly reproduced thus being obtrusive to the viewer. That is, with the present third system, the color of the high luminance portion is excessively outstanding. This is particularly manifest if an object the color of which is predominantly the prime color is imaged.
It is not possible with the second system nor with the third system to change only the saturation of the high luminance portion according to liking, as discussed above.
In view of the above-described status of the art, it is an object of the present invention to provide an image signal processing apparatus whereby only the saturation of the high luminance portion is changed according to the liking, in order to assure more spontaneous luminance compression, and a color video camera having the image signal processing apparatus loaded thereon.
It is another object of the present invention to provide a picture signal processing method whereby only the saturation of the high luminance portion is changed according to the liking, in order to assure more spontaneous luminance compression.
According to the present invention, the above object is accomplished by compressing only the saturation of color image signals represented by three prime color signals so that, in a state in which the color hue and luminance of the color picture signals are maintained, the signal level of the signal of the three prime color signals having the highest signal level will be a first signal level, and by compressing only saturation in a state in which luminance and chromaticity are maintained.
In one aspect, the present invention provides a picture signal processing apparatus including first saturation compression means for compressing only saturation of color image signals represented by three prime color signals so that, in a state in which the color hue and luminance of the color picture signals are maintained, the signal level of the signal of the three prime color signals having the highest signal level will be a first signal level, and second saturation compression means for compressing only the saturation of the color picture signals, compressed in saturation by the first saturation compression means, in a state in which the luminance and the color hue are maintained.
In another aspect, the present invention provides a color video camera including image pickup means for generating three-color signals in meeting with imaging light, first saturation compression means for compressing only saturation of color video signals represented by the three-color signals generated by the image pickup means so that, in a state in which the color hue and saturation of the color video signals are maintained, the maximum signal level of three-color signals will be a first signal level, and second saturation compression means for compressing only saturation of the color video signals, at least compressed in saturation by the first saturation compression means, in a state in which luminance and the color hue of the color video signals are maintained.
In the video signal processing apparatus and the color video camera according to the present invention, luminance compression means may be provided ahead of the first saturation compression means. The signal levels of the input three-color signals may be compressed in signal level with the same compression ratio, in a state in which the color hue and saturation of the color video signals represented by the input three-color signals are maintained so that finally only the saturation of the color video signals will be compressed by the first and second saturation compression means.
Preferably, first and second saturation compression means effectuate saturation compression based on the luminance signal level of color video signals outputted by the luminance compression means. The range of compression of saturation by the second saturation compression means can be variably set by control means.
In yet another aspect, the present invention provides a method for processing picture signals including a first saturation compression step of compressing only saturation of color image signals represented by three prime color signals so that, in a state in which the color hue and luminance of the color picture signals are maintained, the signal level of the signal of the three prime color signals having the highest signal level will be a first signal level, and a second saturation compression step of compressing only saturation of the color picture signals, compressed in saturation by the first saturation compression step, in a state in which the luminance and the color hue are maintained.
In the picture signal processing method of the present invention, it is possible to provide a luminance compression step ahead of the first saturation compression step. By this luminance compression step, the signal levels of the input three-color signals are compressed with the same compression ratio for the input three-color signals, in a state in which the color hue and saturation of the color video signals represented by the input three-color signals are maintained, before executing the first and second saturation compression steps.